The invention relates to high yield production processes for methacrylic acid (“MAA”) and methacrylate esters, such as methyl methacrylate (“MMA”).
A number of commercial processes are used to prepare MMA. In one such process, MMA is prepared from acetone cyanohydrin (“ACH”). The process is described in U.S. Pat. No. 4,529,816 (“816”). In this process, ACH is (1) hydrolyzed by sulfuric acid to produce alpha-hydroxyisobutyramide (“HIBAM”) and its sulfate ester, alpha-sulfatoisobutyramide (“SIBAM”); (2) the HIBAM and SIBAM are thermally converted, or cracked, to 2-methacrylamide (“MAM”) and a small amount of methacrylic acid (“MAA”); which are then (3) esterified with methanol to produce MMA. Residual HIBAM is esterified to methyl alpha-hydroxyisobutyrate (“MOB”). In step (2) of the reaction, the conversion of SIBAM to MAM occurs more readily than the conversion of HIBAM to MAM. In order to facilitate the thermal conversion of HIBAM to MAM, both heat and increased residence time must be provided. A decrease in thermal conversion to desired products results in a decreased overall yield for the process. The process of preparing MAA can be the same as that used to prepare MMA, except that instead of esterifying MAM and MAA with methanol, water is added to the MAM and MAA mixture to convert the MAM to MAA.
One means of employing this chemistry is disclosed in US 20030208093. In that process, one to five hydrolysis reactors are employed to feed an integrated follow-on system that completes the cracking and subsequent steps of the process.
In the ACH hydrolysis process, reaction yield can be lost to the thermal decomposition of ACH to acetone and HCN. In this strongly acidic environment, these immediate decomposition products are also consumed. For example, acetone reacts with sulfuric acid to form acetone mono- and disulfonic acids (AMSA and ADSA) and water. HCN hydrolyzes to formamide, which in turn decomposes to equivalents of CO and NH3 (forming ammonium bisulfate with sulfuric acid).
The ACH hydrolysis process is strongly exothermic; heats of mixing and reaction drive up temperatures, resulting in additional ACH decomposition. Managing this problem requires efficient mixing at the point of ACH introduction to quickly dissipate heat, and cooling to minimize the reaction temperature. The latter, however, is constrained by increasing viscosity and the solidification, or “salting,” of the reaction mixture at low temperatures. The term “salting” is commonly used as it infers the acid-base interactions that take place between sulfuric acid and the amide species present. As a result, ACH hydrolysis processes are typically operated just above the salting temperature for practical management of viscosity and to avoid salting.
While this approach of minimizing hydrolysis temperatures within practical limits is straightforward, it does not provide a complete rationale for improving the overall ACH to MAM process yield, especially for multi-reactor hydrolysis systems. Successful improvements ideally would 1) achieve hydrolysis reactor compositions capable of lower respective operating temperatures and 2) give improved reaction product overall yield through the thermal cracking step. Rationalized paths to these objectives require a thorough understanding of the process/composition/property interrelationships. Unfortunately, these are highly multivariate and, in terms of definitive predictive ability, remain poorly understood.
Ensuring net gains in an MMA process from hydrolysis improvements requires hydrolysis compositions that convert in a net positive overall yield through the thermal cracking step. This latter step, again, converts HIBAM to the desired process intermediate MAM. Due to common HIBAM levels, this step cannot be foregone, yet its conditions are severe and contribute to the bulk of yield losses in the MMA process. Therefore, hydrolysis improvements must strike a balance between value gained from HIBAM versus the other three hydrolysis yield components SIBAM, MAM, and MAA. Hypothetically speaking, an improvement that selectively increases HIBAM hydrolysis yield could result in a reduced overall yield post-thermal cracking.
The MMA and MAA markets are extremely cost sensitive. A slight improvement in process yield can result in a significant market advantage. There is a need for an improved yield commercial process of preparing MMA and/or MAA.